How do gametes differ from normal body cells?
The nucleus of a normal human body cell has 46 chromosomes. Every sperm or egg has only 23 chromosomes. This is because in the testes or ovaries, a process called meiosis takes place. Meiosis is a nuclear division that results in each daughter nucleus containing half the number of chromosomes as the parent nucleus. Usually meiosis takes place in two stages. The final result is that four daughter nuclei or cells are produced. The daughter cells become the gametes.
The set of 23 chromosomes in the gamete is known as the haploid number. Twice the haploid number of chromosomes make up the diploid number. In man, the diploid number is 46. This is the normal number of chromosomes in each body cell of man.
Sexual reproduction involves the fusion of two gametes. During fertilisation, a sperm fuses with an egg to form a zygote. Since the sperm and the egg have the haploid number of 23, the zygote will have a diploid number of 46. The zygote then divides by normal cell division called mitosis, resulting in all the cells in the child having a diploid number of chromosomes except for the ovum and sperm cell.
There are two ovoid testes in the male. The testes lie in a pair of
sacs, called the scrotal sacs, between the thighs. These are pouch-like extensions of the
skin. The development of the sperms require a temperature lower than that of the body.
Therefore, testes lie in the scrotal sacs outside the main body cavity. At the upper end
of each testis is a spermatic cord which contains blood vessels. Leading from the end of
the testes is a narrow, much coiled tube, the epididymis.
The testis produces sperms. Sperms from the testis may be stored temporarily in an inactive form in the epididymis before they enter the sperm duct or vas deferens.
The vas deferens from each side passes into the abdominal cavity. It loops over the ureter of that side and finally opens into the urethra. A gland called the seminal vesicle opens into each vas deferens. At the base of the urinary bladder is the Cowper's gland. The seminal vesicle, the prostate and the Cowper's gland secrete a slippery fluid which mixes with the sperms. The mixture is celled semen. The fluid contains nutrients and enzymes which nourish the sperms and activate them so that they swim actively. The seminal vesicles also store sperms temporarily before ejaculation.
The urethra is a tube which passes through the centre of the penis to the exterior. Semen and urine do not pass through the urethra at the same time. This is done by a circular band of muscle called the sphincter muscle at the base of the urinary bladder. This muscle can be controlled by the will. It prevents urine from coming out of the bladder during ejaculation (release of semen).
The penis is an erectile organ. It contains erectile tissue with numerous blood spaces. When blood spaces are filled with blood the penis becomes hard and erect.
Sperms are produced in vast amounts after puberty period. The sperm or spermatozoon is long and has:
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